In previous studies on immune response in mice infected with M. leprae, attempts were made to evaluate the humoral response. As a parallel to these studies, the cellmediated immune response in similarly infected mice was evaluated, with the idea of placing the mouse model in the spectrum of human leprosy. Identical studies were carried out with mice infected with M. lepraemurium and M. marinum. The studies of M. leprae infected mice indicated that the mouse model can be placed in the BB or BT spectrum of human leprosy whereas the studies on M. lepraemurium infected mice indicated an immune response similar to that exhibited by patients in advanced stages of leprosy infection. The M. marinum studies have shown that this model is not the most convenient one to be used as a parallel for such studies with M. leprae infected mouse model. Studies on the antigenic mosaic of acid-fast bacilli isolated from human leproma, have shown no similarity between these organisms and M. leprae. Isoelectric focusing, a highly sensitive separation technique, have yielded fractions from M. smegmatis CF and CE, that appear to have single antigen. Similar studies are in progress on the M. leprae CE and other mycobacterial preparations.